Move and Sew UV Edges combines separate UV shells along their selected border edges by moving one selected UV shell (the smaller one) to the other selected UV shell (the larger one) and merging the selected edges together so that one UV shell results. Move and Sew UV Edges is useful for quickly joining together separate UV shells produced by Automatic Mapping or the Layout UVs features. This feature is also available from the toolbar of the UV Texture Editor.
There are two options for moving and sewing UV shells: manually or automatic. The default Move and Sew UV Edges behavior is to manually select the edges you want to join and then execute the feature. When the Limit shell size option is turned on, smaller shells are moved and sewn automatically to the larger ones based on the Number of faces setting value.
Edit UVs > Move and Sew UV Edges > 
When Limit shell size is on, this setting specifies the maximum number of faces in each shell. Larger numbers result in fewer shells.
After performing a Move and Sew operation, you can select the history node (polyMapSewMove) in the Channel Box and adjust the Number of Faces until you achieve the results you want.